The present invention relates to a xenon-metal halide lamp having an improved electrode structure which is particularly suited for forward lighting applications of a vehicle such as an automobile, truck, bus, van or tractor. More particularly, the improved electrode structure comprises a shank having a coil wrapped about it for accurately aligning the electrodes to the axis of the lamp. The improved electrode has parameters that are selected to accommodate the various electrical current conditions which occur during the operation of the xenon-metal halide lamp.
A xenon-metal halide lamp serving well as a light source for an automotive headlamp is disclosed in U.S. patent application Ser. No. 157,436 of Bergman et al. filed 02/18/88 assigned to the same assignee as the present invention and herein incorporated by reference. The light source of Bergman et al. contains a xenon gas which provides for the instantaneous light needed for automotive applications along with mercury and metal halide ingredients that provide for the high efficiency lumen output of the automotive headlamp.
In optical systems such as automotive headlamps, it is desired that the source of light be accurately located relative to the reflector of the headlamp. In automotive headlamps using discharge light sources, such as a xenon-metal halide lamp, it is desired that the arc be located between the electrodes so as to serve as a light source that is accurately located relative to the envelope comprising the light source itself. One of the means of accomplishing such locating of the arc is to accurately center the electrodes within the envelope. Various schemes to achieve electrodes centering are known. For example, the shaped foil described in U.S. Pat. No. 4,254,356 of Karikas provides the means to fit the electrodes into quartz tubing, forming a light source, and holding the electrodes on the axis of the envelope. The means of Karikas serves well the needs of lamps having a relatively long distance or arc gap between electrodes. However, for lamps with short arc gaps, such as between 1.5 to 3 mm as for xenon-metal halide lamps used for automotive applications, the shaped foil of Karikas does not provide sufficiently accurate and repeatable centering of the electrodes. Furthermore, for low wattage lamps, the bulb size should be very small in order to obtain high efficiency. To obtain consistent performance, that is color and efficiency, from lamp to lamp it is necessary that the electrodes be accurately positioned on the axis of the lamp. It is desired that means be provided to more accurately allow centering of the electrodes to be accomplished so that the optical position of the light generated by the xenon-metal halide may be more precisely known.
A further consideration related to the electrodes of the xenon-metal halide lamp is the different amounts of current the electrodes must carry during the various phases of operation of the xenon-metal halide lamp. The various phases of a xenon-metal halide lamp, as somewhat described in U.S. patent application Ser. No. 157,436, may be considered as; (1) the initial starting phase in which light is produced by the excitation of the xenon gas which requires a relatively high current to produce sufficient power because the voltage drop through the lamp is relatively low (15V) to form an electron emitting spot to be created on the electrode at a low voltage (2) the phase of mercury vaporization with increase in voltage drop and the warming up of the electrodes to a full thermionic state, and (3) the final or run phase of operation in which the vaporization and excitation of the metal halides in addition to the emission of the mercury supplies the steady state light output of the lamp.
In order to obtain an efficient light output during warm-up of the high pressure xenon-metal halide lamp, which includes initial and intermediate phases of lamp operation, a current several times higher than the normal or run current is commonly desired. This heavy current requires that an electrode have dimensions that are much heavier than would be required if only the lower run current was needed. The heavy dimensions are required so that the electrode has sufficient current carrying capabilities so as to not melt or vaporize during the warm-up phase of lamp operation. This same electrode must, however, run sufficiently hot so as to maintain thermionic emission and thereby operate stably during the steady state operation which occurs at a much lower current. It is desired that the electrodes have parameters that accommodate the various current needs related to the operation of the xenon-metal halide lamp.
Accordingly, it is an object of the present invention to provide an electrode structure that is adapted to the different amounts of current occurring during the various phases of operating the high pressure xenon-metal halide lamp.
It is a further object of the present invention to provide means for accurately centering the electrodes of the xenon-metal halide lamp.